Method of Making High Strength Ferrous Article, and Article Made Thereby

ABSTRACT

A preselected metallurgical phase is formed in a body of a ferrous alloy by heating the workpiece to a temperature above an austenizing temperature of the alloy and thereafter contacting the workpiece with a quenching medium. Heat is input to the workpiece during at least a portion of the time it is in contact with the quenching medium. The quenching medium and/or the source of heat are regulated so that they work in combination to maintain the workpiece at a holding temperature which is above the temperature at which the martensite phase forms in the alloy. After maintaining the alloy at the holding temperature for a predetermined period of time, it is cooled to ambient. In particular implementations, the system operates to form a bainite phase in the alloy. Also disclosed are systems for implementing the method.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. ______ filed Sep. 15, 2005, entitled “Method of Making HighStrength Bainite Article, and Article Made Thereby.” This applicationalso claims priority of U.S. Provisional Patent Application Ser. No.60/610,720 filed Sep. 17, 2004, entitled “Metal Forming Process withResistance Heating.”

FIELD OF THE INVENTION

This invention relates generally to the fabrication of articles fromferrous metals having controlled metallurgical properties. Morespecifically, the invention relates to the fabrication of articles fromferrous alloys having high concentrations of a bainite phase. Mostspecifically, the invention relates to the fabrication of structuralmembers for motor vehicles, and other articles of manufacture,fabricated from high strength steel having a high concentration of abainite phase.

BACKGROUND OF THE INVENTION

Ultra high strength steel is used to fabricate various components ofmotor vehicles, building structures, and other articles of fabrication.In typical processes of the prior art, such articles are fabricateddirectly from martensite steel or they are fabricated from a low carbonsteel which is converted to martensite during the production process. Ineither instance, steel products produced thereby have tensile strengthsup to and over 200 ksi. However, such martensite structures arecompromised in elongation properties (typically 4-6%). Bainite steels,particularly lower bainite steels, have very high strength but also havevery good elongation properties, typically approaching 15%. The superiorelongation properties greatly enhance the utility of such articlesand/or simplify their fabrication.

While bainite steels are advantageous in many applications, thepreparation of steels having a significant bainite phase, and inparticular a lower bainite phase, is difficult. In order to form thebainite phase, the steel alloys must be first heated to a temperatureabove their austenizing temperature then quenched to a particularholding temperature which is above the temperature at which themartensite phase forms, and they must be held at this temperature forrelatively long periods of time so that the bainite phase may form.Lower bainite is formed when the steel is held at a temperature closeto, but above, the temperature at which martensite forms. The holdingstep requires both the combination of fairly high temperatures and verygood temperature control. As a consequence, this step is typicallycarried out in relatively large volumes of high temperature fluids suchas molten salts or heated oils. These high temperature baths presentsignificant hazards, consume relatively large amounts of energy, andoccupy relatively large areas.

As will be explained in detail hereinbelow, the present inventionprovides a process whereby ferrous articles having precisely controlledmetallurgical properties may be fabricated in a process having a highlysimplified holding step which eliminates the use of high temperaturebaths and the like, but still promotes the formation of selectedmetallic phases such as upper, lower or mixed bainite phases.Accordingly, the process of the present invention may be readily adaptedto the large volume, high speed fabrication of high strength steelalloys articles such as intrusion beams, frame members, body members andthe like for motor vehicles, as well as components of buildingstructures and other articles of manufacture.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed herein is a method for preparing a body of a ferrous alloycontaining a high percentage of a desired metallic phase or phases. Theinvention is described with particular reference to the formation ofbainite phases; but, it may be utilized to form articles having otherdesirable metallurgical phases therein. In a first step of the method, aworkpiece comprised of a ferrous alloy is heated to a temperature abovean austenizing temperature of the alloy. Thereafter, the workpiece iscontacted with a quenching medium. Heat is input to the workpiece atleast during a portion of the time that it is in contact with thequenching medium so that the combination of the input heat and thequenching medium cooperate to maintain the workpiece at a holdingtemperature which is above the martensite phase formation temperature ofthe alloy. The workpiece is maintained at this holding temperature for aperiod of time sufficient to form a desired phase therein and thereafterthe workpiece is cooled to ambient temperature. In particularembodiments of the present invention, the combination of the holdingtemperature and time are selected so that a bainite phase is formed inthe workpiece. In specific instances, the thickness of the workpiece isno greater than 5 millimeters, and in some certain instances it is nogreater than 3 millimeters.

In some instances, the workpiece can be heated by resistance heatingwherein an electrical current is flowed therethrough. The quenchingmedium may comprise a gas or a liquid, and it may be flowed across,sprayed onto, or flowed through the workpiece, while in other instances,the workpiece may be immersed in a bath of the quenchant fluid. In someinstances, the workpiece may be subjected to a forming operation whicheither changes or maintains its shape. This forming operation may beimplemented either prior to heating above the austenizing temperature,while the workpiece is at the austenizing temperature, while theworkpiece is at the hold temperature, or after the process is complete.Also disclosed herein are articles made by the process of the presentinvention as well as apparatus for carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing one heating profile which may be implementedin accord with the present invention to produce a lower bainite phase ina workpiece; and

FIG. 2 is a diagram of a temperature control circuit which may beutilized in the practice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a metallurgical process which greatlysimplifies the formation of bainite, or other desirable metallurgicalphases, containing articles by decoupling the function of the quenchmedium from that of the hold medium heretofore employed. As such, thepresent invention eliminates the need to utilize large, hightemperature, liquid baths for holding articles at an elevatedtemperature during the formation of the desired phase.

The present invention relies upon a combination of input heat and aquench medium to dynamically balance the temperature of a workpiece soas to effectively hold that workpiece at an elevated temperaturesufficient to promote the formation of a bainite or other desired phase.

In a typical process of the present invention, a workpiece comprised ofa ferrous alloy is first heated to a temperature above its austenizingtemperature. This heating may be accomplished by any means known in theart. For example, heating may be carried out in a furnace, by aninductive heater or by resistive heating where an electrical current isflowed through an article. Once the article is heated to this initialtemperature, it is then contacted with a quench medium. This quenchmedium may comprise a simple fluid such as water or a water-basedliquid, an oil or the like, or in some instances it may be a liquefiedor vaporized gas. The quench fluid may be in the form of a bath in whichthe article is immersed, or it may comprise a sprayed volume of fluid.In any instance, the fluid need not be at a very high temperature sinceheat will also be input to the workpiece. The quench fluid may comprisea gas including ambient air, an inert gas such as nitrogen, argon or thelike, or a reactive gas such as a nitriding or carburizing gas. Duringat least a part of the time that the workpiece is in contact with thequenching medium, it is also being heated. The input of heat working incombination with the quench medium establishes a dynamic equilibriumwhich maintains the workpiece at a selected temperature. By controllingthe parameters of heating and quenching, the workpiece may be veryaccurately maintained at a holding temperature selected so as to promotethe formation of the desired phase. The balance of heat and cooling ismaintained for a period of time sufficient to form this phase, afterwhich the heat input is terminated and the workpiece cooled below atransition temperature so as to preserve the desired phase. As will beappreciated, this process is highly controllable. In a typical operationfor forming lower bainite, the workpiece is maintained at a holdingtemperature which is no greater than 350° C. but above the temperatureat which the martensite phase begins to form. The precise temperatureranges will depend upon the specific alloys being treated. In someinstances the alloy will be held at temperatures above 350° C.,depending on the phase being formed.

Heat input during the hold phase may be implemented by various processesknown in the art. One particular process which may be advantageouslyemployed in the present invention comprises a resistive heating whereinan electrical current is flowed through the article so as to generateheat. The resistive heating process is economical and simple toimplement and control. In some instances, the article may be initiallyheated to a temperature above the austenizing temperature by a resistiveheating process preferably carried out in the absence of a quench mediumand thereafter contacted with a quench medium so as to cool it to thehold temperature. Such heating and holding may be advantageously carriedout in a single workstation and can be coordinated with further formingprocesses carried out before, during or after the hold step.

Other heating processes such as inductive heating, flame heating,radiant energy heating and the like may also be employed in the practiceof the present invention. In some instances it may be advantageous tomeasure the temperature of the workpiece during the hold step, andtemperature information obtained thereby may be utilized to control theinput of heat and/or parameters of the quench medium such astemperature, velocity, pressure and the like as appropriate, so as toallow for accurate temperature control. Such control may be carried outin a feedback mode or in an indirect mode.

Referring now to FIG. 1, there is shown a time versus temperatureprofile for one process in accord with the present invention as operableto form a lower bainite phase in a ferrous body. The graph of FIG. 1plots time along the horizontal axis and temperature along the verticalaxis. At the beginning of the process, the workpiece is at ambienttemperature, which is understood to be a normal room temperatureencountered in the workplace; and in any instance, an ambienttemperature is a temperature which is sufficiently low so thatsignificant metallurgical transitions will not occur in the workpiece.Typically, ambient temperatures are below 50° C. In a first portion ofthe process, the workpiece is heated to a temperature which is above theaustenizing temperature of the alloy comprising the workpiece. Thistemperature will vary dependent upon the particular alloy employed;however, one of ordinary skill in the art could readily determine whatthis temperature should be. This first stage heating is typicallycarried out fairly rapidly, although this is not a requirement of theinvention. Heating may be in a furnace, in which instance parts may bemaintained therein until further processed. In other instances, heatingmay be carried out on individual parts immediately before processing.Heating can be by flame, induction, resistance heating or any othermethod available to those of skill in the art. Once the workpiece israised to a temperature above the austenizing temperature, it is thencooled to a holding temperature which is typically below 350° C. butabove the temperature at which a martensite phase will form in thealloy. The part is cooled from the austenizing temperature to theholding temperature by application of a quenchant fluid thereto. Duringat least a portion of the time that the workpiece is in the hold phase,heat is applied thereto in conjunction with the quenchant fluid. Thisheating may be by any means known in the art; however, because of easeof application and control, resistance heating is one preferred heatingmethod. Induction heating is another. The combination of heat input andquenchant will establish a dynamic equilibrium, as noted above, whichmaintains the workpiece at the hold temperature. The application of heatmay be continuous or it may be intermittent as is necessary. Theworkpiece is held at the hold temperature for a period of timesufficient to form desirable amounts of the bainite phase; andthereafter, it is cooled to an ambient temperature thereby locking inthe bainite phase. Cooling is typically accomplished by the quenchantfluid, and additional heat is not input in most instances.

As will be apparent to those of skill in the art, the aforedescribedheating profile may be modified depending upon the specific metallurgyof the alloy being employed and/or in accordance with desired processingparameters. For example, the rate at which the workpiece is heated orcooled may be varied. Likewise, the hold phase, while shown as being asingle horizontal line, may be a stepped line or a gradually slopingline. In some instances, temperature spikes may be selectivelyincorporated into the profile. And as noted above, the hold temperaturemay be above 350° C. when other phases are being formed.

Also, it should be noted that the present invention allows for very goodspatial control of the metallurgical properties of a workpiece. Forexample, by appropriately configuring the heating and quenching deliverysystems, portions of a workpiece may be selectably heated, held andcooled so as to preferentially form a bainite phase in certain portionsof a workpiece. In this manner physical parameters of a workpiece suchas deformability may be optimized for particular applications.

Referring now to FIG. 2, there is shown a schematic depiction of onetemperature control circuit which may be implemented in the presentinvention. The circuit of FIG. 2 operates to maintain a workpiece 10 ata preselected temperature. In that regard, a temperature sensor 12 is incommunication with the workpiece and is operative to produce an outputsignal indicative of its temperature. The temperature sensor 12 maycomprise a thermocouple, a thermometer, a thermistor or some other suchcontact device. Also, it may comprise a non-contact device such as anoptical pyrometer. In any instance, the temperature sensor 12 produces acontrol signal which is communicated to a quench controller 14 and/or aheater controller 16. In the illustrated embodiment, the temperaturesensor communicates with both the quench controller and the heatercontroller; but in some instances, sufficient temperature control can beobtained in the workpiece by controlling only a single one of thecontrollers. As illustrated, the quench controller 14 controls a quenchfluid delivery system 18 which supplies a quench fluid to the workpiece10. Similarly, the heater controller 16 controls a heater 20 whichinputs heat to the workpiece. As will be appreciated, the controllers14, 16 can cooperate to heat and cool the workpiece 10 in accord with apreselected profile. One of skill in the art can readily incorporateother embodiments of controller in the system of the present invention.

Most preferably, the methods of the present invention are implementedutilizing workpieces which are relatively thin, and in that regard havea thickness of no more than 5 millimeters; and in some particularinstances, the thickness of the articles is no more than 3 millimeters.It has been found that very good temperature control, and uniformmetallurgical properties can be obtained utilizing articles of thisdimension. However, thicker articles may be fabricated by appropriatelyadjusting the temperature and heat transfer capacity of the quenchmedium. The method of the present invention is very well adapted to thefabrication of relatively thin sheet metal articles such as bumperbeams, intrusion beams, frame members, body panels and the like formotor vehicles. It may also be utilized to fabricate panels, beams,braces and similar components of building structures. The method of thepresent invention may be implemented in conjunction with other formingprocesses such as roll forming, stamping, bending, die forming and thelike. As such, stations and systems for the formation of the bainitephase may be incorporated directly into various apparatus, particularlyin those instances where processes are carried out at elevatedtemperatures. For example, a bainite formation step may be readilyincorporated into hot forming operations such as die forming, heattreating and the like.

In summary, the present invention operates so as to balance heating andcooling and thereby allow a workpiece to be processed according to aprecisely controlled temperature profile. This allows for very preciseand accurate selection and control of the article's metallurgicalproperties.

In view of the disclosure presented herein, numerous modifications andvariations of the present invention will be readily apparent to those ofskill in the art. The foregoing is illustrative of specific embodimentsof the invention, but is not meant to be a limitation upon the practicethereof. It is the following claims, including all equivalents, whichdefine the scope of the invention.

1. A method for preparing a body of a ferrous alloy containing a highpercentage of a preselected metallic phase, said method comprising thesteps of: providing a workpiece comprised of a ferrous alloy; heatingsaid workpiece to a temperature above an austenizing temperature of saidalloy; and thereafter contacting said workpiece with a quenching medium;inputting heat to said workpiece during at least a portion of the timesaid workpiece is in contact with said quenching medium so that thecombination of said heat which is input, and said quenching mediumcooperate to maintain said workpiece at a preselected holdingtemperature which is above the martensite phase formation temperature ofsaid alloy; maintaining said workpiece at said holding temperature for aperiod of time sufficient to form a preselected metallic phase therein;and thereafter cooling the workpiece to an ambient temperature.
 2. Themethod of claim 1, wherein said workpiece is held at a combination ofsaid holding temperature and said time so as to form a bainite phasetherein.
 3. The method of claim 1, wherein said temperature is below350° C.
 4. The method of claim 2, wherein said bainite phase is a lowerbainite phase.
 5. The method of claim 2, wherein said bainite phase isan upper bainite phase.
 6. The method of claim 1, wherein the thicknessof said workpiece is no greater than 5 millimeters.
 7. The method ofclaim 1, wherein the thickness of said workpiece is no greater than 3millimeters.
 8. The method of claim 1, wherein the step of inputtingheat to said workpiece comprises flowing an electrical current throughsaid workpiece so as to resistively heat said workpiece.
 9. The methodof claim 1, wherein the step of heating said workpiece to a temperatureabove an austenizing temperature comprises flowing an electrical currentthrough said workpiece so as to resistively heat said workpiece.
 10. Themethod of claim 1, wherein the step of contacting said workpiece with aquenching medium comprises contacting said workpiece with a liquidmedium.
 11. The method of claim 1, wherein the step of contacting saidworkpiece with a quenching medium comprises contacting said workpiecewith a gaseous quenching medium.
 12. The method of claim 1, wherein thestep of contacting said workpiece with a quenching medium comprisesimmersing said workpiece in said quenching medium.
 13. The method ofclaim 1, wherein the step of contacting said workpiece with a quenchingmedium comprises spraying said quenching medium onto said workpiece. 14.The method of claim 1, including the further step of measuring thetemperature of said workpiece while it is in contact with said quenchingmedium and said heat is being input thereinto.
 15. The method of claim13, including the further step of controlling the input of heat and/or aparameter of said quenching medium in response to the temperature ofsaid workpiece.
 16. The method of claim 1, wherein the step of inputtingheat to said workpiece comprises inductively heating said workpiece. 17.The method of claim 1, including the further step of carrying out aforming operation on said workpiece.
 18. An article made by the methodof claim
 1. 19. An apparatus for preparing a body of a ferrous alloyhaving a high percentage of a preselected metallic phase, said systemcomprising: a quenchant delivery system operable to deliver a quenchantfluid to a workpiece; a heater operable to heat the workpiece during atleast part of the time the quenchant delivery system is delivering saidquenchant to the workpiece; and a controller for controlling at leastone of said quenchant delivery system and said heater, so that saidquenchant delivery system and said heater cooperate to maintain theworkpiece at a preselected temperature which is above the martensitephase formation temperature of said alloy.
 20. The apparatus of claim17, further including a temperature sensor for generating a signalcorresponding to the temperature of said workpiece, said temperaturesensor being further operable to communicate said signal to thecontroller.